Magneto-electric machine



April 7, 1942. G. w. RAWLINGS I MAGNETO-ELECTRIC MACHINE 3 Sheets-Sheet l Filed May 10, 1957 5 4 VOLTS 21 24 27 CYCLEMPH.

m m m mm m m 9% Inventor? B1 hisAttor'ngys April 7, 1942. G. w. RAWLINGS MAGNETO-ELECTE KIC MACHINE Filed May 10, 1937 3 Sheets-Sheet 2 1B 27 CYCLEHPH. 2000 3000- 4000 5000 6000 7000 8000 Dvnmo RPM. 1

24 27 CYCLE HP".

DYMHO mm.

I nvenior'a By his Attorney:

April 7, 1942. G. w. RAWLINGS 2,278,489

MAGNETO-ELECTRIC MACHINE Filed May 10, 1957 3 Sheets-Sheet 3 Fla Inven tor? By his Attorney:

UNITED I STATES PATENT' OFFICE I macuaro-ift a zirrc moms Gar... William Bawlings, Kenilworth, England (Ci. iii-209) 2 claims.

This invention relates to magneto-electric generators, that is to say, dynamos incorporating a permanent magnet element, and more particularly to such machines of the kind comprising a one piece multi-polar permanent magnet field magnet encircling a multi-polar armature, that is to say, such having at least 4 magnetic poles.

The primary object of the invention is to provide a simple means of obtaining load voltage/speed regulation combined with high specific output in a small dynamo suitable for use in the electric lighting system of a cycle or motor cycle.

The expression load voltage/speed is used herein to mean the relation between the speed and the voltage generated under load, as compared with the relation on open circuit, that is to say, the relation as may be represented by a graph plotting voltage readings against speed.

In such .an electric lighting system one of the mostdesirable features of the dynamo is that its output at the lower end of the speed range shall increase rapidly with the speed up to the rated or standard output, after which any'increase in speed shall not be accompanied by any substantial increase in output with voltage use.

In a dynamo possessing this desirable characteristic the rated output is reached at a comparatively low speed, and is maintained substantially constant thereafter despite even a large increase in speed. For example, such a dynamo of 6 volts 6 watts rating would begin to light a 6 volt 6 watt bulb at 3 cycle miles per hour, and would reach its 'rated output at 11 or 12 cycle miles per hour, and yet would not unduly overload the bulb at 30 cycle miles per hour.

There are other uses than as a cycle lighting dynamo for which the aforesaid characteristics are desirable in magneto-electric machines.

It is the purpose of the present invention to provide a new form of encircling permanent magnet construction in dynamos as aforesaid to obtain in combination the characteristic of high specific output and load voltage/speed regulation from armature reaction or back ampere turns" effect.

I have discovered that, in a dynamo, the extent to which a given armature reaction flux effectively opposes the contrary fiux of a permanent magnet of given strength and so-eifects load voltage/speed regulation, depends upon the inter polar contour of the encircling permanent magnet. This is particularly noticeable in dynamos of high specific output. When the encircling magnet has an uninterrupted circular bore the armature reaction load voltage/speed regulationg' effect is found to be at a maximum; conversely,\ when the encircling magnet has large and deep intervals between the polar projections the armature reaction efiect is at a minimum, so that in a dynamo of high specific output as aforesaid the load voltage/speed characteristic under given conditions can be determined between limits by the inter polar contour of the encircling permanent magnet.

It is believed that this effect is due to the fact that in the magnet with the uninterrupted circular bore, or small depth of inter-polar space more of the armature reaction flux is constrained to pass through the field magnet and thus usefully oppose or restrict the available flux of the field magnet, whereas in the field magnet with deep spaces between polar projections, the armature reaction fiux being opposed by the field magnet fiux, more of such reaction fiux can and does pass through the air space between the polar projections and, therefore, has less effect on the field magnet flux and is, therefore, less eflective as a means of load voltage/speed regulation. Of course this method of voltage regulation does reduce the specific output and this must be allowed for, but ahigh rated output is still obtainable, especially at the slower speeds.

I have also discovered that, by using the modern alloys of high coercivity, the optimum lengthsection ratio for the field magnet may be substantially retained while bringing the contour of the inner polar portions of the magnets close to the armature tunnel and thereby more effectively placing the magnet in the path of the armature reaction flux.

This effect of load voltage/speed regulation is 7 only practically obtainable in machines having four or more poles both on the magnet and on the armature since where there are fewer than four poles the armature reaction flux, owing to the negative susceptibility of the magnet, mostly passes between the tips of the armature poles through the already comparatively deep air space defined by a chord between the said pole tips and the wall of the armature tunnel so that the armature reaction flux is not constrained to pass through the magnet and oppose the normal flux therein.

The expression negative susceptibility is used to indicate the inherent opposition of already magnetised metal to the entrance and passage of an opposite magnetic fiux.

The above theoretical deductions together with the aforesaid discoveries obtained by practical present invention.

According to the invention, a magneto-electric machine of the kind having an armature encircled by a one piece ring permanent magnet with at least four magnetic poles of alternate polarity or a magnet therefor is characterised in that the inner contour of the magnet between such poles is either an uninterrupted cylindrical bore forming the armature tunnel or an interrupted bore comprising alternate polar projections and interpolar spaces, which spaces are collectiveiy less than 25% of the volume of the metal of the magnet.

The drawings illustrate more or less diagrammatically certain embodiments of the invention in which:

Fig. 1 shows diagrammatically the field magnet and armature of a dynamo;

Fig. 2 is a side elevation of the field magnet shown in Fig. 1.

Figs. 3 and 4 show forms of continuous ring magnets having polar projections and interpolar spaces;

Figs. 5, 6 and 7 are graphs showing the characteristics of the dynamos employing respective- 1y the field magnet shown in Figs. 1, 3 and 4.

Figs. 8 and 9 show another form of field magnet particularly suitable for a slow speed dynamo.

Fig. 10 shows a flux diagram illustrating a means of determining the contour of an interpolar space.

The characteristic curves shown in Figs. 5. 6 and 7 indicate still further the method whereby voltage regulation is eilected by determining the inner contour of the magnet between the poles. As used in these figures, US and A/S indicate voltage-speed curve and current-speed curve re spectiveiy. These curves show the characteristics oi a typical dynamo when fitted alternatively with the magnets shown in Figs. 1, 3 and 4. These three magnets are designed to be made from magnet material 01 high coercivity such as the nickel aluminium alloys having a coercive force of 500 Gilberts per centimeter. The optimum dimensional relation for such metal is about 3/1, therefore the magnet is designed to have one unit in section and I2 units in mean circular length. As there are 4 poles the length of each magnet compared with its section is 3/1, so that the metal is used under optimum conditions, i. e., to give the maximum, magnetic strength/weight ratio. This ratio is substantially maintained in all the examples herein described.

The magnet shown a in Fig. 1, has an uninterrupted circular bore a, that is to say, with no defined polar projections. but it is magnetised to have four magnetic poles as indicated. The magnet shown in Fig. 3. is formed with arched inter polar spaces d rising to 1*." beyond the armature tunnel, whilst the magnet e shown in Fig. 4, has a still more defined arched contour between the polar projections. and this is the preferred form.

The dynamo with which the three magnets shown in Figs. 1, 3 and 4 were used alternatively to obtain the characteristic curves shown in Figs. 5, 6 and 7, was constructed with a laminated 4-.poie armature of the form shown I) in Fig. l, the armature being wound with 160 turns per pole of 26 s. W. G. enamelled copper wire, giving 640 total turns. Each field magnet was permanently magnetised, prior to test, in position in the machine by passing momentarily through the armature windings a current of 40 amperes. The

experiment form the basis of development of the magnetising force so engendered was sumcient to magnetise each magnet to saturation.

It will be observed that by this method of magnetising the magnet with its own armature, the most eilective flux concentration is obtained. Also, the fact that the inter-polar spaces of the field magnet are designed to reduce armature fiux leakage makes the armature an ei'i'icient means for "flashing" the magnet.

The characteristic curves shown in Figs. 5, 6 and '7 were obtained from the generator in circuit in each case with a bulb of 6 volts 6 watt rating.

It will be observed from comparison of the curves shown in Figs. 5, 6 and 7, that under the same conditions the extent of the load voltage/speed regulation consequent upon armature reaction is determined by the inter polar contour of the encircling magnet. With a magnet having an uninterrupted circular bore as shown in Fig. l, the reaction effect is at a maximum due, it is believed, to the fact that under these conditions the maximum of armature reaction fiux is constrained to pass through the encircling magnet. and so most effectively to restrict or oppose the contrary flux of the permanent magnet. When the encircling magnet is provided with arched inter polar spaces as shown in Figs. 3 and 4, the armature reaction is less eifective as a means of load voltage/speed regulation, and accordingly within the limits of a given machine the load voltage/speed characteristic under a given load can be determined as required by determining the form or the inter polar contour of the encircling permanent magnet.

In the case of the magnet a shown in Fig. 1 having an uninterrupted circular bore a forming the armature tunnel, the reaction effect was so severe that under full load the magnet was found tosufl'er'permanent demagnetisation and loss of pole definition or "pole shift." This condition was not experienced in the case of the magnets shown in Figs. 3 and 4. and it will be appreciated that between the first extreme condition wherein the reaction effect demagnetises the magnet, and the opposite extreme condition wherein the reaction effect has very little infiuence in limiting the voltage, there is determinable a condition wherein the output characteristics obtained are suited to the purpose required.

The field magnet shown in Figs. 8 and 9 illustrates a practical form of a magnet for a slow speed multi-polar dynamo embodying the load voltage/speed determination feature of the invention.

The spaces 01 the permanent rings of Figs. 3, 4 and 8 are so formed that the depth of the rings as measured in a radial direction at the spaces is considerably less than the depth of the rings as measured in a radial direction at the polar projections. In Figs. 4 and 8 the depth of the rings at the spaces is about 66 per cent of the depth of the rings at the polar projections and in Fig. 3 the depth oi the ring at the spaces is about per cent of the depth of the ring at the polar pro- Jections.

Fig. 10 shows a theoretical iiux diagram of armature reaction flux. The lines of magnetic fiux illustrated follow the recognised theory of crossing equi-potential planes at right angles, assuming that they remain in the same medium, such as air. From the pole centres there is shown the radial dividing lines X, whilst A represents the flux path from the centre of each half pole. B represents the flux path commencing from points half way between the half pole centres and the pole tips and subtending an angle of about 56 degrees at the center and is the preferred limit of shaping, beyond which effective voltage/speed control is lost. The shaded portion between the armature tunnel and this line B represents the area within which the inter polar shape effects the loadvoltage/speed characteristic, according to the present invention, it being assumed in this example that the pole width is equal to the width of the inter polar space. The line C represents a suitable inter polar shape for the field magnet, such line lying within the shaded area. As can be seen, the volume of the interpolar spaces represented by the shaded area is lessthan 25% of the volume of the metal of the magnet.

' In explanation of the results obtained in practice and as shown in the graphs, the theory is advanced, as already stated, that the smaller the air space formed within the shaded area the more is the armature reaction flux compelled to neutralise or oppose the flux of the permanent magnet instead of passing through the air space of the inter-polar space, and the more is such armature reaction able to control the load voltage/speed characteristic.

It is further to be. observed that the comparatively narrow poles in the magnet of Fig. 4 are adopted to minimise the distortion of the flux path due to the rotation of the armature. It .is suggested that the more concentrated the poles the less the disturbance at the surface of the molecular construction. The extent to which thepole surface can be reduced however is limited by considerations of the flux density in the air gap, which in turn affects the output.

The armature lamination form shown is a compromise as regards length of pole tip; the

tip should be as long as possible to bridge the poles and avoid open-circulting the magnet, but

on the other hand the length mustbe restricted to facilitate winding by machine. In actual practice the armature does not actually bridge the pole tips but the material for the magnet is of such high coercivity that there is no appreciable loss from open-circuiting to this extent.

A ring magnet having four'poles has already been proposed built up of segmental lamination,

the segments overlapping at the poles only and having air spaces between them intermediately of the poles, such air spaces forming parts of the interpolar spaces and in'which that part of eachinterpolar space formed in a segment was itself within the boundary of the sub-mean armature reaction flux path in air. The advantages obtained by constructing the magnet according to the present invention invention would, however, not be obtainable with this previously proposed construction since, the armature reaction flux would not be constrained to pass through the magnet itself but would, owing to the negative susceptibility of the magnet, pass through the air spaces between the laminations without acting to oppose the main flux in the magnet and thus voltage control would not be obtained.

What I claim is:

1. A dynamo which comprises a continuous ring permanent magnet having at least four magnetic poles of altemate' polarity, an armature having the same number of poles as the ring magnet, the inner contour of the ring magnet comprising alternate polar projections and" arched interpolar spaces, the interpolar contour of each interpolar space being substantially along an arched line connecting the inner extremities of the polar projections, the formation of the magnet between the polar projections conforming to the natural path of the armature reaction flux in air, so that the reaction flux effectively restricts the flux of the permanent magnet, the dynamo having the characteristics of load voltage/speed regulation combined with high specific output.

2. A dynamo which comprises a continuous ring permanent magnet having at least four magnetic poles of alternate polarity, the inner contour of the ring magnet comprising alternate polar projections and interpolar spaces, the contour of each interpolar space being substantially coincident with an arched line connecting the inner extremities of the adjacent polar projections and conforming generally to an armature reaction flux path; in air connecting the same extremities, said armature reaction flux path being located between the armature poles in air when the ring magnet is removed.

GEORGE WILLIAM RAWHNGS. 

